Improvement of flight distance is a permanent theme of golfers. Therefore, in the design of golf club heads, various plans have been devised in order to improve the rebound performance of golf club heads. On the other hand, when the position of the center of gravity of a golf club head is lowered, for example, the position of a sweet spot can be brought closer to a face center, and when striking a ball, the ball is more easily caught in the vicinity of the sweet spot, resulting in an expected improvement in flight distance. Therefore, a lower center of gravity is often sought in the design of golf club heads.
Incidentally, a golf club head having a plurality of grooves formed in a sole portion is disclosed in U.S. Pat. No. 8,517,860.